Reactive distillation for esterification reactions is known. In WO 01/27065 for example, a reactive distillation process for the preparation of ethyl acetate is disclosed wherein ethanol is reacted with acetic acid and/or acetic anhydride in the presence of a solid acidic catalyst. Acetic acid or anhydride or their mixture and ethanol are introduced separately into the reactive distillation column.
In U.S. Pat. No. 6,028,215, a process for preparing esters from alcohol and carboxylic acid, in particular butyl acetate from butanol and acetic acid, is disclosed wherein an alcohol and a carboxylic acid are reacted in the presence of a heterogeneous acid catalyst.
A disadvantage of a reactive distillation process for the esterification of carboxylic acids with lower alcohols is that, in case the alcohol is not entirely consumed, a water/alcohol mixture is distilled over the top of the column. Recovery and recycling of the alcohol requires additional separation steps, usually one or more further distillation columns.
In a paper in Chemical Engineering and Processing 43 (2004) 411-420, Dimian et al. describe the use of a mass separation agent, usually called entrainer, in a reactive distillation process for the catalytic esterification of fatty acids with light alcohols that form homogeneous azeotropes with water. It is shown that adding an entrainer to the top of the distillation column overcomes limitations due to distillation boundaries. The entrainer enhances water removal from the reaction zone and, as a result, water can be separated from the top stream by simple decantation. In this paper the use of n-propyl-acetate as entrainer in the esterification of lauric acid with 1-propanol is exemplified. Hydrocarbons, both aliphatic and aromatic, oxygenates such as esters, ethers and ketones, and halogenated hydrocarbons are mentioned as suitable entrainers.
In EP 1 300 387, a process for the manufacture of an ester of lactic acid by reactive distillation over a heterogeneous catalyst is disclosed. An entrainer is used to separate water introduced with the reactants and water produced by the esterification reaction from the distillation column. The distillation column is filled with reactant alcohol and entrainer before start-up of the reaction. During the reactive distillation, additional entrainer may be added. It is not mentioned to which part of the column further entrainer is added.